1. Field of the Invention
The invention pertains to communication and sensing systems which include waveguides, such as optical fibers.
2. Art Background
Waveguides are devides used for guiding electromagnetic radiation. Included among such devices are optical fibers, which typically include a core and a cladding, and are generally fabricated from silica. Because the core is conventionally fabricated to have a higher refractive index than the cladding, electromagnetic radiation is largely confined to, and guided by, the core through the phenomenon of total internal reflection.
Optical fibers are now being used, or have been proposed for use, in a wide variety of optical communication and sensing systems. In the former category, one of the most promising applications of optical fiber is in the role of an optical data bus linking the input/output (I/O) devices, e.g., computer terminals, of a local area network (LAN). (A LAN is a multiple-access communication system in which two or more I/O devices, such as computer terminals, are linked to a central processing unit and/or to each other through a data link.) That is, each I/O device of an optical fiber LAN includes an optical source and an optical detector for, respectively, generating and detecting optical signals, the optical fiber data bus serving to communicate these optical signals between the I/O devices.
An optical fiber LAN must necessarily include optical taps for tapping optical signals out of the optical fiber data bus and into each I/O device, and optical couplers for coupling optical signals from each I/O device into the optical fiber data bus. A variety of taps are now in use, including invasive taps (taps which require cutting into the optical fiber data bus) and noninvasive taps. The latter are generally preferred to avoid degrading the strength of the data bus. Exemplary noninvasive taps include one or more small bends (typically having a radius of curvature of several centimeters) in the optical fiber data bus, which results in optical signal power being radiated out of the data bus.
An exemplary, currently used optical coupler, useful both for coupling optical signals into, and for tapping optical signals out of, an optical fiber data bus includes two optical fibers 10 and 20 (see FIG. 1). Typically, the coupler is formed by excising a sufficiently large cladding portion from each of the two fibers so that when the resulting fiber surfaces are contacted, the two optical fiber cores are brought into relatively close proximity (to within a few core diameters or into contact). Then, the two cores are carefully aligned with each other to achieve a desired optical coupling efficiency by sliding the two optical fibers along their cut surfaces. Finally, the cut surfaces are fused together through, for example, a heat treatment. Coupling an optical signal from an I/O device into an optical fiber data bus is then achievable by cutting the optical fiber data bus, and splicing one of the fibers of the optical coupler, e.g., the optical fiber 10, into the cut portion of the data bus. That is, upon launching an optical signal from an I/O device into the optical fiber 20, a portion of the evanescent field associated with this optical signal will extend into, and be guided by, the core of the fiber 10, and thus be coupled into the core of the optical fiber data bus. (The evanescent field is the exponentially decaying portion of the electromagnetic radiation guided by an optical fiber which extends beyond the core, and into the cladding, of the fiber.)
While the above-described optical coupler is usefule, it does have a number of drawbacks. For example, the process of joining the coupler to an optical fiber data bus is invasive, i.e., requires the bus to be cut, which necessarily degrades the physical integrity of the bus. In addition, the fabrication of the optical coupler (which, in use, becomes an integral part of the optical fiber data bus) effectively constitutes an additional step in the fabrication of the optical fiber data bus. As a consequence, the complexity, and cost, of manufacture of the optical fiber data bus is effectively and substantially increased. Moreover, the addition of each new user to a LAN involves the splicing of an optical coupler to the optical fiber data bus (a process typically performed after installation of the optical fiber data bus), which generally requires a service interruption. Significantly, the two splices involved in joining an optical coupler to an optical fiber data bus typically exhibit a combined optical loss of about 0.5 dB. In addition, a portion of any optical signal guided by the optical fiber data bus is necessarily coupled into each optical coupler (including those for which the signal is not intended), with each such coupling typically constituting a loss of about 0.5 dB. (There is also an additional loss associated with each optical coupler, also typically about 0.5 dB, which, as yet, is unexplained.) Thus the addition of each new user produces a significant, and cumulative, increase in loss. In fact, the splice, coupling, and other (as yet, unexplained) losses are so high that the total number of LAN users is typically limited to no more than about 20.
As noted, optical fibers have also been proposed for use in sensors. For example, one type of fiber, potentially useful as a distributed sensor (a sensor capable of detecting stimuli at a plurality of spaced points or regions), is a plastic clad silica (PCS) fiber. When used as a distributed sensor, a PCS fiber typically includes a silica core 30 (see FIG. 2) as well as a plastic cladding 40 containing distributed active centers such as neodymium or oxazine perchlorate. When subjected to an external stimulus, e.g., a change in ambient temperature or a change in the concentration of a chemical, the active centers respond by changing their optical absorption. Thus, by transmitting those wavelengths of electromagnetic radiation whose intensities are likely to be affected by this change in absorption, and by detecting the intensities of these wavelengths, the presence or absence of a stimulus is readily determined. Moreover, by reflecting these wavelengths back through the fiber to their point of origin, and using the conventional technique of optical time domain reflectometry (OTDR), the position of the stimulus along the length of the fiber is also readily determined. (Regarding OTDR see, e.g., M. Barnoski and S. Personick, "Measurements in Fiber Optics," Proceedings of IEEE, Vol. 66, No. 4, pp. 429-441 (1978).)
Although active center-doped PCS fibers are certainly useful as distributed sensors, they are limited in that their spatial resolution, i.e., their ability to resolve the spatial extent of a stimulus, is no better than about 50 meters. Consequently, the location of stimuli can only be determined to within about 50 meters. In addition, the OTDR apparatus is very expensive (typically costing between ten and twenty thousand dollars).
Thus, those engaged in the development of optical fiber systems have sought, thus far without success, optical fiber LANs in which the optical couplers are formed without degrading the physical integrity of the optical fiber data bus, without increasing the complexity of manufacture of the optical fiber data bus, without the need for loss-producing splices, and without service interruptions. In addition, the developers of optical fiber systems have also sought, thus far without success, optical fiber distributed sensors which are both relatively cheap and have spatial resolutions significantly smaller than about 50 meters.